Magnetic Resonance Imaging at 7T Reveals Common Events in Age-Related Sarcopenia and in the Homeostatic Response to Muscle Sterile Injury

Abstract

Skeletal muscle remodeling in response to various noxae physiologically includes structural changes and inflammatory events. The possibility to study those phenomena in-vivo has been hampered by the lack of validated imaging tools. In our study, we have relied on multiparametric magnetic resonance imaging for quantitative monitoring of muscle changes in mice experiencing age-related sarcopenia or active regeneration after sterile acute injury of tibialis anterior muscle induced by cardiotoxin (CTX) injection. The extent of myofibrils’ necrosis, leukocyte infiltration, and regeneration have been evaluated and compared with parameters from magnetic resonance imaging: T2-mapping (T2 relaxation time; T2-rt), diffusion-tensor imaging (fractional anisotropy, F.A.) and diffusion weighted imaging (apparent diffusion coefficient, ADC). Inflammatory leukocytes within the perimysium and heterogeneous size of fibers characterized aged muscles. They displayed significantly increased T2-rt (P<0.05) and F.A. (P<0.05) compared with young muscles. After acute damage T2-rt increased in otherwise healthy young muscles with a peak at day 3, followed by a progressive decrease to basal values. F.A. dropped 24 hours after injury and afterward increased above the basal level in the regenerated muscle (from day 7 to day 15) returning to the basal value at the end of the follow up period. The ADC displayed opposite kinetics. T2-rt positively correlated with the number of infiltrating leucocytes retrieved by immunomagnetic bead sorting from the tissue (r = 0.92) and with the damage/infiltration score (r = 0.88) while F.A. correlated with the extent of tissue regeneration evaluated at various time points after injury (r = 0.88). Our results indicate that multiparametric MRI is a sensitive and informative tool for monitoring inflammatory and structural muscle changes in living experimental animals; particularly, it allows identifying the increase of T2-rt and F.A. as common events reflecting inflammatory infiltration and muscle regeneration in the transient response of the tissue to acute injury and in the persistent adaptation to aging.

Myeloid HIFs Are Dispensable for Resolution of Inflammation during Skeletal Muscle Regeneration

“…These findings clearly showed that myeloid HIFs are not essential for skeletal muscle regeneration. Although the CTX-induced MRI changes observed in this study are in line with those recently reported in the literature (33), we further demonstrated that the increase of FA occurring 7 d after injury (33) was actually related to the decreased l 2 and l 3 values, thereby reflecting the well-known presence of small-diameter regenerating muscle fibers (22). Although it has been suggested that T 2 variations represent a marker of muscle inflammation (33,34), it should be pointed out that a linear relationship of T 2 and intracellular volume has been previously reported (53).…”

“…Although the CTX-induced MRI changes observed in this study are in line with those recently reported in the literature (33), we further demonstrated that the increase of FA occurring 7 d after injury (33) was actually related to the decreased l 2 and l 3 values, thereby reflecting the well-known presence of small-diameter regenerating muscle fibers (22). Although it has been suggested that T 2 variations represent a marker of muscle inflammation (33,34), it should be pointed out that a linear relationship of T 2 and intracellular volume has been previously reported (53). On that basis and considering the reduced second and third eigenvalues at day 7, the associated higher T 2 values might actually be related to the expansion of the extracellular component, including, for instance, a widened interstitial space (54,55), rather than to intracellular changes.…”

“…In brief, recent studies demonstrated that T 2 changes can be used as an index of muscle inflammation (33,34) even though other biological phenomena (edema, necrosis) can also affect T 2 values (38). In addition, diffusion-weighted MRI provides information about the water molecules self-diffusion, which is restricted by physical barriers such as membranes, cytoskeleton, mitochondria, and sarcoplasmic reticulum, thereby leading to anisotropic diffusion.…”

“…Therefore, in line with a previous work showing in vitro that HIF-1a and HIF-2a mRNA are differentially expressed in M1 and M2 MPs (32), we investigated whether HIFs play a role at the time of MP phenotype skewing toward M2 profile and later during muscle regeneration and healing. We originally combined in vitro investigations with in vivo multimodal magnetic resonance imaging (MRI), including transverse relaxation time (T 2 ) mapping and diffusion tensor imaging (DTI), which has been recently introduced for noninvasive quantitative monitoring of the different physiological steps associated with skeletal muscle injury and regeneration (33,34).…”

Leukocyte HMGB1 Is Required for Vessel Remodeling in Regenerating Muscles

“…Magnetic resonance imaging (MRI) acquisition has been performed using a 7-T horizontal 30-cm bore, 300.3-MHz unit (BioSpec 70/30 USR, ParaVision 5.0; Bruker BioSpin, Karlsruhe, Germany) (23).…”